This invention relates to shearing apparatus and, more particularly, to apparatus for severing bar or rod stock, such as bone pins or other elongate elements, along a plane transversely of the longitudinal axis without imposing twisting or other forces upon the element during severing.
Heretofore, devices for shearing rod stock such as bone pins and similar other cylindrical elements, which are often formed of stainless steel or titanim alloy having extreme hardness and thus difficult to cut, have included saws, abrasive cutters, manually operated shears and the like, which may be satisfactory for usage in some instances with rods of small diameter, but are difficult to use, and frequently unacceptable in other applications requiring the shearing of hard, larger diameter rods or pins. For example, in certain orthopedic applications, such as the fixation of fractures of large bones, in which the intramedullary bone pins typicaly have a diameter of one-fourth to three-eighths inch, manually operated shears are unsatisfactory because of the length of the handles required to develop the forces necessry to shear a rod of this diameter. Furthermore, an open cutting mechanism such as a shear tends to bend rather than cut the member, the bending not only increasing the area of the plane of shear and increasing the force required to sever the member but also resulting in dissipation of energy in effecting the bending which otherwise would contribute to the shearing force. Among the practical disadvantages of manually operated shears, even if the handles can be made sufficiently long to enable cutting of larger diameter rods, is that they frequently leave a jagged and distorted end after severing, an unacceptable result in orthopedic applications. Known rotatable pin cutters, although causing less bending than open shears during the severing operation, also frequently leave a jagged end after severing and are otherwise unsatisfactory for many cutting operations. Devices of this type comprise a rotatable pin cutter plug positioned within a cutter body, the body and cutter plug being relatively rotated by movement of handles attached to each of the elements. The rod or pin to be sheared is placed into a passageway in the cutter plug, which is initially aligned with an aperture in the cutter body, and shearing is accomplished by manually rotating the elements with respect to each other. When rods of relatively large diameter are involved, long handles are required to develop sufficient force to shear the rod, making the device cumbersome and under some circumstances unusable, as for severing a pin already implanted in the bone of a patient.
Among the applications in which abrasive cutters are used to sever rod or bar stock is the cutting of test pieces for tensile strength and/or other analysis of the bar material. However, the heat generated by this cutting method may sufficiently alter the surface characteristics of the material being severed as to preclude an accurate analysis of the material in an uncut rod. An efficient shearing device dissipates much less heat than does an abrasive cutter, sufficiently less that the surface characteristics are not appreciably affected, making shearing an acceptable technique for the cutting of test specimens and creating the need for efficient and conveniently usable shearing apparatus.
A shearing device described in U.S. Pat. No. 3,972,116, designed primarily for the shearing of bone pins and which overcomes the described disadvantages of manually operated shears and rotatable pin cutters, employs relative translational movement between a cutter and a cutter body. The described shear comprises an interfitting assembly of an elongated cutter body having an interior channel in which a cutter plug is slidably mounted. The body member has apertures intersecting the internal channel for receiving a pin and the cutter plug has a pin passageway therethrough. In a first longitudinal position of the cutter plug, perparatory to receiving the pin, the aperture and passageway are aligned and in a second position they are out of registry. The body member encloses the cutter plug to provide a close interfit between them whereby a cylindrical element, such as a bone pin, placed in the apertures and passageway in the first longitudinal position of the plug is severed by movement of the cutter plug to the second position; the apertures and passageway are sized to closely interface with the peripheral surface of the pin so as to substantially eliminate the tendency of the pin to bend at the point of shear. Relative movement of the cutter plug and cutter body is produced by a pneumatically operated mechanism which may include means for force multiplication.
Although the apparatus disclosed in U.S. Pat. No. 3,972,116 is effective in minimizing bending at the point of shear, its use is limited to those applications in which the pin to be sheared has a free end for insertion into the aligned aperture and passageway; thus, the device is unusable in situations where the rod or other elongated element, such as spinal rods which are implanted or fixed at both ends and which sometimes have to be cut to effect an adjustment. The problem of severing an elongated element having both ends fixed is also found in industrial applications, such as interlocked netting of the kind used for the reinforcement of concrete formed of relatively large diameter rods, or the standing rigging of sailboats, or steel cable. Thus, there is a need for shearing apparatus capable of severing or cutting rigid metal rods or pins the ends of which are fixed or otherwise inaccessible which, at the same time, does not have a tendency to bend the rod at the point of shear. Moreover, such shearing apparatus should be relatively small and compact, yet rugged, so as to be capable of severing bone pins, or other rod elements, in locations where working space is limited.
The shearing apparatus of the present invention utilizes the effective shearing properties of the shear described in U.S. Pat. No. 3,972,116 by, in effect, replacing the apertures in the body member with an open slot formed in the wall of the body member for receiving the rod to be severed, and replacing the passageway in the cutter plug with an open-sided rod-receiving slot. However, these changes, without more, would result in the rod not being sufficiently enclosed to prevent the tendency of the rod to bend at the point of shear as the cutter plug is moved relative to the cutter body. In other words, if the position of the apertures and cutter plug passageway were displaced sufficiently to form an open slot in a wall of the patented cutter (a modification not suggested in the patent), approximately half of the periphery of the rod received in the slot would be unenclosed, thus allowing it to bend under the extreme forces present during the shearing action.
It is an object of the present invention to provide shearing apparatus essentially having the shearing properties of the described prior art shear, yet capable of severing a rod or other elongate element whose ends are inaccessible for insertion in the shear, without racking or twisting the element. Another object of the invention is to provide a shear that is relatively small and compact yet rugged, and capable of severing rods or pins in locations where working space is limited.